1. Field of the Invention
The present invention relates to an information recording and reproducing apparatus for an optical disk.
2. Description of the Prior Art
Hitherto, an apparatus is known wherein a DC track groove which is .lambda./8 in depth is previously formed on a disk, a tracking signal is detected by utilizing the fact that asymmetry in diffracted light beam is caused by the edge of the track groove when the light spot gets out of the center of the track, and the information is recorded on the DC track groove by the laser light which is modulated by an information signal while a track is being traced by using the above-described tracking signal. In this apparatus, two photodetectors arranged in parallel with the track receive diffracted light beam and a tracking signal is detected by deriving the difference between the outputs of these two photodetectors. Such an apparatus has been disclosed in the Japanese Patent Application Laid-Open No. 60702/74.
If the light spot in this apparatus is moved by a galvanomirror, for example, for tracking control, a tracking offset is produced in the tracking signal. Further, the coma aberration is produced when a disk is tilted. Due to this coma aberration, especially two photodetectors which are arranged in parallel with the track become out of balance, generating more track offsets. Accordingly, normal track tracing is not conducted. This tracking signal is represented by the function of the DC groove on the disk, i.e., the depth and width of the groove, wavelength and distribution of the light beam and the numerical aperture of the lens.
FIG. 10 shows a tracking signal obtained when a light spot gets out of the center of a DC groove in a conventional apparatus.
As for the parameters of the disk and optical system, the wavelength .lambda.=0.82 .mu.m, groove depth=[/8, groove width=0.45 .mu.m, track pitch=1.6 .mu.m, numerical aperture of lens=0.5, distribution coefficient .gamma..sup.2 =4, and .theta.=0.5.degree.. As evident from FIG. 10, the tracking offset is 0.08 .mu.m in this case. Experiments shows that the offset as much as 0.08 .mu.m loses the stable tracking control.
Further, the conventional apparatus has the following problems. When a disk having the information recorded at high density on its DC groove is reproduced, the tracking offset is increased due to a lowered tracking sensitivity. In addition, in a conventional DC groove system having a groove .lambda./8 deep, the selectable range of recording material is narrowed and the application range is limited. That is to say, the tracking signal becomes unstable by the irregularity of the reflectivity, unevenness or the irregularity of the refractive index of the disk.